In recent years, an LED (Light Emitting Diode) has been used as a light source used in a backlight of a liquid crystal display device, in place of a CCFL (Cold Cathode Fluorescent Lamp) employing a fluorescent bulb.
Particularly, in terms of easiness in controlling balance of colors, a method of obtaining a white color by (i) using each of primary colors of a red LED, a green LED, and a blue LED, independently, and (ii) optically combining the colors in an additive manner is advantageous, and therefore a lot of research has been made in order to apply the method to a television technology.
Basically, an LED has a property of having a change in its luminance depending on a current, and a forward voltage of an LED varies depending on individual differences, and/or the temperature, for example. For this reason, in a case where an LED is used as a backlight of a liquid crystal panel (an LCD (Liquid Crystal Display), for example), it is demanded that a drive device for driving the LED has a constant current characteristic so as to obtain a constant and uniform luminance.
As a brief explanation, as illustrated in FIG. 8, such a method has been known that an LED 102 and a resistance element 103 are connected to an output of a constant-voltage power source 101 in series so as to control a current. However, in a device illustrated in FIG. 8, with a high-luminance LED 102 in which a high current flows, the resistance element 103 causes a lot of power loss.
In order to solve the problem, there has been a method of using a constant current power source 201 as a drive device, as illustrated in FIG. 9. With the drive device, it is possible to adjust a luminance of the LED 102 by lessening (reducing) a current value. In order to change the current value, the following method has been generally used. That is, (i) a resistance element 105 is inserted in series with the LED 102, (ii) a current value is detected from a potential difference between both ends of the resistance element 105, and (iii) feedback control is carried out so as to cause the current value to be a desired value.
However, lower the current becomes, lower the potential difference becomes. This reduces a degree of accuracy in detecting the current value, and tends to cause the detection to be affected by a noise or the like. Further, if a high resistance value is set for the purpose of obtaining a sufficient voltage from a low current, there is a disadvantage that, in a case of a high current, power loss becomes large.
In order to solve the problem, a drive device adopting a PWM control method has been known. In the PWM control method, in order to stably adjust the luminance in a wide dynamic range, a current of an LED is turned on or off at certain timing, and the luminance is adjusted depending on a ratio of the on state to the off state.
As one of methods for realizing the PWM control method, the following method has been adopted. That is, as illustrated in FIG. 10, a switch element 106 is inserted in series with the LED 102, and is turned on or off by use of a PWM signal (PWM control signal) at predetermined timing.
As described above, if a drive device, such as a backlight of an LCD or an electric light, adopts the PWM control method with which a light emitting element is turned on or off at predetermined timing so as to adjust the luminance based on the ratio of the on state to the off state, it becomes possible to stably adjust the luminance in a wide range.
Meanwhile, a frequency of the PWM control is usually set to be not less than 60 Hz so as to avoid a flicker.
However, magnetic members (not illustrated) (such as a transformer (not illustrated) and a choke coil (note illustrated)), a capacitor (not illustrated), and the like, each of which is used in the constant voltage power source 101, fundamentally have a property of vibrating in accordance with a frequency of an applied current and/or an applied voltage. The frequency that is not less than 60 Hz is included in a human audible range. Therefore, there is a disadvantage that a human audible noise often occurs.
On the other hand, Patent Literature 1 discloses a parameter technique of setting a frequency of a PWM signal to be not less than 20 kHz. By setting a frequency of a PWM control signal to be not less than 20 kHz, it becomes possible to cause the vibration generated by the magnetic members (such as the transformer and the choke coil), or the capacitor, to be a frequency not less than 20 kHz. Therefore, it becomes possible to prevent a human audible noise from being generated.